Method for reacting to disturbance in a mobile communication system

ABSTRACT

A method for reacting to disturbance on an uplink frequency in a mobile communication system which comprises a mobile communication network which includes at least one base station and a mobile services switching center, as well as at least one mobile station which communicates with base station on an uplink, i.e. receive, frequency, and on a downlink, i.e. transmit, frequency of base station. In order to react advantageously to an uplink disturbance, the following procedure will be followed: a mobile communication network detects the disturbance on an uplink frequency, the mobile communication network informs a mobile station of disturbance by using a downlink frequency and commands the mobile station to listening mode in which the mobile station stays on the same downlink frequency and receives information from the base station, the mobile station shifts to listening mode.

This application claims benefit of international application PCT/FI95/00318, filed Jun. 5, 1995.

FIELD OF THE INVENTION

The invention relates to a method for reacting to disturbance on anuplink frequency in a mobile communication system which comprises amobile communication network which includes at least one base stationand a mobile services switching center, as well as at least one mobilestation which communicates with the base station on an uplink, i.e.receive, frequency, and on a downlink, i.e. transmit, frequency of thebase station.

The invention relates to mobile communication systems and especiallytrunked mobile radio systems which comprise control centers, basestations as well as mobile stations, typically radiotelephones, andwhich systems are cellular, and which system has in each cell one ormore radio channels on certain frequencies and which channels arepossibly split into time slots. In each cell, i.e. base station, one ormore radio channels are used for signalling, while the rest are intraffic channel use.

BACKGROUND OF THE INVENTION

Present mobile communication systems have a feature according to whichmobile stations do not accept a control channel which does not respondto their signalling messages. In cases wherein mobile stations do notreceive a response to messages they have transmitted to base stations,they begin to search for a new control channel to use. Furthermore, inthe present mobile communication systems, mobile stations are not at allinformed of failures in the uplink communications, nor of reasonscausing these failures, but they themselves simply notice that a basestation and/or a system is not able to respond in a sensible manner tomessages and signalling transmitted by the mobile station.

In such a case, as a mobile station notices that signalling between itand the system is not working, the mobile station cannot always in allcases independently infer the reasons causing the break. If a basestation transmission is interpretable at a mobile station, but themobile station does not receive any response to its signalling, themobile station can only identify that the failure occurs in the uplinktraffic. Uplink communication is so called, because it is carried out onthe receive frequency of a base station.

In cases wherein a mobile station is in normal mode although uplinkcommunication does not work, the phone can not at all utilize thoseservices of a radio system that require acknowledgements or uplinksignalling. If, for example, a mobile station is turned on in theservice area of such a base station whose uplink radio frequency,typically system frequency or channel, does not work, and the systemrequires a successful registration of the mobile station to the systembefore any actual user information can be transmitted, the mobilestation is completely useless to its user, as the base station is notable to receive registration messages transmitted by the mobile station.The same holds true in cases, a mobile station moves from coverage areasof other base stations to an uplink disturbance area. In such a case, amobile station such as a radiotelephone, cannot update its locationinformation to the system and, thus, is not able in any way to utilizethe system services. In such a case, the mobile station begins to scanthe frequencies and channels of other base stations in an effort to finda new base station. It is possible that a handover and accomplishswitching to another base station, i.e. roaming, succeed, or it may alsohappen that the base station is not able to find a working base stationand its control channel at all, and, as a result, is not able to utilizethe services of the mobile communication system.

FIG. 1 shows a situation in which the transmitting power of a mobilestation MS is considerably lower than that of a base station BS. In sucha case, there may be a prior break in the uplink connection due todifferences between up/downlink coverage areas. The figure shows thecoverage area of a base station BS by curve C1, and the coverage area ofa mobile station MS by curve C2. The aforementioned means that thetransmission of the base station BS carries further than from wherevehicle-mounted and hand-held mobile stations MS can successfullytransmit a radio signal to the base station BS. In this prior artexample, the base station does not hear the mobile station MS althoughthe mobile station MS hears the base station BS, and so the mobilestation MS begins to search for a new channel, for example, a controlchannel. If other channels are available, the mobile station moves tothem and drops the original call, but if other frequencies or channelsare not available, the mobile station cannot utilize the services of themobile communication network.

In the prior art analog PMR systems (PMR=Private Mobile Radio),criminals have been able to listen to police communications withoutauthorization, and, thus, it has not been necessary to jam the radiotransmitter. If, however, such analog, open channel uplink traffic of amobile communication system or a mobile radio system has been jammed,the mobile stations have remained on the channel whose uplink frequencyis jammed and have been able to receive commands from the systemdispatcher on a downlink frequency.

In the prior art trunked systems, which are often networks used by theauthorities, for example, police radio networks, a high-grade encryptioncan be implemented, which makes it impossible to listen to trafficbetween mobile stations and base stations. It is, however, certain thatthese systems will be jammed by criminals in order to, for example,hinder the police command communications. Technically, it is much easierto jam an uplink than a downlink frequency. As the systems used by theauthorities only have a limited number of frequencies, a situation canemerge in which a certain base station can only use one frequency, andthe signals of other base stations do not reach all parts of the servicearea of that certain base station. Furthermore, it is easy to build ajammer which monitors the used frequencies by scanning. For thesereasons, disturbance elimination mechanisms that are based on channelcirculation are ineffective.

The operation of a radio system can be jammed by a jamming transmitter,by of transmitting on the frequencies in use a radio signal which ismore powerful than the utility signal. Jamming can occur both on thetransmit (downlink) frequency and the receive (uplink) frequency of abase station. Close to a base station, the downlink radio signal is farstronger than the uplink signal, because of the high transmitting powerof a base station, and because the radio signal transmitted by the basestation is not significantly attenuated. The level of an uplink radiosignal transmitted by a mobile station is lower even at transmission,and, depending on the location of the mobile station, the signalstrength may be considerably attenuated close to the base station. Thus,it is much easier to jam an uplink than a downlink signal by a jammer.In practise, the jamming of an uplink signal is the only way to try tohinder the operation of a mobile communication system by way of blockingradio frequencies.

If the spacing of transmit and receive frequencies, the so-calledduplex-spacing, i.e. the spacing of uplink and downlink frequencies, isconstant, it is possible to efficiently block all the uplink traffic ofthe base station by an intelligent jammer. An intelligent jammermeasures on which downlink channel the base station is transmitting andimmediately blocks the corresponding uplink frequency by a jammingtransmission, the uplink frequency being at a duplex-spacing distancefrom the transmit frequency. In such a case, it does not help that thebase station changes its transmit/receive frequency if it suspectsjamming, because the new frequency will be jammed, too.

FIG. 2 illustrates a possible scenario of jamming of a base station. Agang of criminals robs a bank in an urban area, and they know thelocation of the base station BS of the police radio network, and set ajammer N on the uplink frequency F1 of the base station BS. The jammeduplink frequency F1 or channel is inoperative, but the correspondingdownlink F2 is not. There are police units in the coverage area of thebase station BS, and additional units are ordered to the crime scenefrom elsewhere. The network dispatcher notices a break in uplinkfrequency F1 in the crime area. The situation may be such that themobile communication system is economically planned so that the radiocells only have a sufficient enough overlap, i.e. the coverage areas ofthe base stations overlap as little as possible, and because the crimearea is not in the overlap zone, the mobile stations cannot search for achannel to use in the coverage area of some other base station.

In the scenario above, the usual analog, so-called open channel system,functions in the following way: the roaming of mobile stations MS, i.e.handover, is carried out by a manual handover performed by the mobilestation user. In an open system, no registration of a mobile station MSto the system is required. In such a case, although the uplink frequencyF1 or channel is jammed, the dispatcher can transmit commands to mobilestations MS in the area on the downlink channel or frequency F2 even ifit does not receive a response from them. In an open channel system,then, a break in the uplink frequency F1 does not influence the downlinktraffic F2. Thus, an open channel has a good jamming resistance.

In the scenario above, a digital trunked system without the operation ofthe present invention functions as follows. At least an adequate levelof security is required of systems used by the authorities, i.e. it isrequired that mobile stations MS which roam, i.e. switch to a new basestation BS, register or at least update their location information tothe system. So, in the scenario above, as the registration and locationupdating requests do not reach the base station BS due to a break in theuplink frequency F1, the mobile stations MS do not accept the channel,but try to search for other control channels of which there are none,because the mobile stations MS are not located in the overlap zone ofthe base stations BS. As no registration is carried out, the mobilestations MS cannot in any way communicate with the system. The situationabove is naturally a serious fault in systems used by the authorities.

Another possible factor blocking uplink traffic is a breakdown of thebase station BS receiver or of the mobile station MS transmitter.

SUMMARY OF THE INVENTION

The aim of this invention is to avoid the problems of the prior artsolutions described above.

The aim is to offer a method according to which a mobile communicationsystem can function in the best possible way in cases the functioning ofan uplink frequency or channel of a base station breaks due to, forexample, disturbance or a technical fault. The aim is to make itpossible that mobile stations on the downlink frequency of the basestation can still receive information from the system, and that othermobile stations possibly entering the coverage area of the base stationcan utilize the services of the mobile communication network.

This new kind of method for reacting to disturbance in a mobilecommunication system is obtained by the method of the invention whichmethod is characterized in that it comprises the following steps: amobile communication network detects the disturbance on an uplinkfrequency, the mobile communication network informs a mobile station ofthe disturbance by using a downlink frequency and commands the mobilestation to listening mode in which the mobile station stays on the samedownlink frequency and receives information from the base station, themobile station shifts to listening mode.

The invention is based on the idea that as disturbance occurs on anuplink frequency of a base station of a mobile communication system,i.e. on the receive frequency of the base station, the system, forexample, a base station, informs the mobile station or radiotelephonesof the break in the uplink traffic and, if desired, also the reason forit by a failure report message. In the same context, the mobilecommunication system orders the mobile station to listening mode. Inlistening mode, a roaming sequence, i.e. the sequence of a base stationchange and a handover sequence, alter. In such a case, the mobilestation exceptionally stays tuned to the downlink frequencycorresponding to the jammed uplink frequency although it does notreceive any responses to its signalling because the base station isunable to hear the mobile station due to the disturbance. It should benoted that the mobile station which is requested to shift to listeningmode can also behave in another way than to shift directly to listeningmode. In a failure report message, a mobile station can also berequested to behave differently. In listening mode, the mobile stationdoes not have to respond to the signalling of the system. In such acase, the services which use both-way traffic of the system in itsnormal mode can be used as one-way services. The mobile station detectson the basis of the failure report message, or lack of it, the reasonfor the uplink break and informs the user of it, who can react in anappropriate way on the basis of that report.

An advantage of this kind of a method for reacting to disturbance in amobile communication system is that the disadvantages of the prior artsolution are avoided by this method.

The present invention provides the advantage that a mobile communicationsystem can detect a disturbance situation on an uplink frequency andreact to it in the best possible way.

In addition, it is an advantage of the present invention that a mobilestation, after it has received a failure report message, shifts tolistening mode in which it can receive signalling and user informationfrom the system on the downlink frequency without the normally requiredboth-way signalling with the system. If the mobile station does notreceive a failure report message as it detects the break, it can inferthat the mobile equipment is faulty, or the break is caused by aninadequate uplink service range, and inform the user of these factors.

It is an advantage of the method of the invention that the mobilestation user is always aware of the reason causing the uplink failure.For example, if the failure is caused by jamming, it is not necessaryfor the user to try to replace his or her unit with a properly workingone, because he or she will know that the failure is caused by jamming.

In addition, it is an advantage of the present invention that not onlycan listening mode be used during a break in the uplink connection butalso in cases only dowlink traffic is required due to operational needs.In such a case, the power consumption of mobile stations can be reduced,and, for example, wireless silence can be ensured.

It is, furthermore, an advantage of the present invention that it iseasier to locate the jammer, as the jammer is the only transmittingdevice apart from the base station in cases the mobile stations are inlistening mode.

In addition, it is an advantage of this invention that, if followed, theservices of the mobile communication system in its normal mode can beutilized in the downlink direction even if the uplink frequency ofchannel were useless.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in closer detail withreference to the accompanying drawings, in which

FIG. 1 shows the coverage areas of the transmitters of a base stationand a hand-held mobile phone in a situation in which the base stationhas a larger coverage area than the hand-held mobile phone, this figurehaving been discussed in the Background section of this document,

FIG. 2 shows a jamming situation in the operation of a radio system, inwhich situation the uplink frequency is jammed, and in which a jammermeasures the downlink frequency and transmits a jamming signal on anuplink frequency which is at a duplex-spacing distance from the downlinksignal 1, this FIG. having been discussed in the Background section ofthis document,

FIG. 3 shows a system diagram of a situation in which a mobile stationcommunicates with a base station,

FIG. 4 shows a block diagram of a failure report message of theinvention, and,

FIG. 5 shows a flowchart of operation of a mobile station of theinvention during jamming.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

FIG. 3 shows a system diagram of a situation in which a mobile stationcommunicates with a base station. The figure shows a trunking systemcontroller TSC to which a base station BS is connected by data links. Amobile station MS is tuned to the downlink frequency F3 of the basestation. There is disturbance on the uplink frequency or channel F4between the base station BS and the mobile station MS.

As the mobile communication system notices a break in the uplinktraffic, it tries to infer the reason for the break. It can be jamming,a breakdown of a radio receiver or some other piece of equipmentrequired in the reception, or some other reason. The break may affectthe entire system or just a certain radio frequency or channel.

Here, a break in the uplink traffic detected by the system is understoodto refer to a break which affects the mobile stations in the coveragearea of the base station.

The system can detect disturbance in the uplink traffic by measuring thestrength of the received signal and comparing it to the bit error ratio.The radio system recognizes that the strength of the received signal andthe bit error ratio depend on each other. The mobile communicationnetwork, i.e. a mobile station or a mobile services switching center,can detect a jamming transmission if a bit error ratio of a signalreceived at the base station at a predetermined field strength exceedsthe corresponding bit error ratio of a corresponding field strength inan undisturbed situation. A digital demodulator can also detect jammingon the basis of different than normal behavior of the received signal.

According to the invention, a mobile communication system couples theinformation on the break and the reason for it to a broadcast messagewhich it transmits on its downlink frequency or channel in order toinform all or some of the mobile stations of it. This information iscalled a failure report message. FIG. 4 shows a block diagram of afailure report message of the invention. The failure report message canalso be signalled to the radiotelephones by unit/group specificsignalling instead of, or in addition to, broadcast signalling.

The fields of a failure report message:

41. message identifier: the system failure report message

42. address of a mobile station such as a mobile station or a group

43. failure description: no disturbance, or jamming or a failure in thebase station receiver or other disturbance

44. mode command: listening mode or normal mode

45. test field

A failure report message can either be sent continuously or more seldom.The message includes a mode command field which determines the mode ofthe radiotelephones.

As uplink breaks occur, the mobile station may shift to listening mode.The command to shift to listening mode is included in the failure reportmessage. In listening mode, the system does not require any kinds ofacknowledgements from the mobile stations. Registrations are no longerperformed, and signalling only takes place in the downlink direction. Insuch a case, the services of the mobile communication system in itsnormal mode can be used in the downlink direction. It is also possibleto act in such a way that the mobile station user himself or herselfupon receiving a request to shift to listening mode gives a manualcommand that switches the mobile station to listening mode. Beforeswitching a mobile station to listening mode, the right of a mobilestation to shift to listening mode can be checked.

As the mobile station in the search sequence of a calling channel hasaccepted a calling channel and moves onto that, the mobile station readsthe identifier information from the calling channel. On the basis ofthis information, the mobile station knows if it has to update itslocation information, or not. It is useless for the mobile station totransmit location area updating information to the system in adisturbance situation, because there is disturbance on the uplinkfrequency or channel of the base station. So, the mobile communicationnetwork, i.e. the base station, the trunking controller or the mobilecommunication system does not acknowlegde anything to the mobilestation. The present, prior art, mobile stations retransmit a failedsignalling for a few times. Thus, the mobile communication system has tobe familiar with the retransmission algorithm, and it has to retransmita failure report message often enough so that the mobile station doesnot have time to begin to search for a new calling channel. Thetransmission rate of a failure report message naturally depends on thelength of the time-out which controls the change of a mobile stationonto a new frequency. If it is unnecessary for the mobile station toupdate its location area, the phone receives a failure report messagewithout problems as it listens to a control channel transmitted by thejammed base station.

The following describes the operation of a mobile station which is inlistening mode. The mobile communication system determines the mode ofthe mobile station (listening mode or normal mode). The mobile stationfunctions in a predetermined mode in the mode command field 44 of thefailure report message. The mobile station user can be informed of theswitch to listening mode and of its reasons by text, sound, and/orindicator lamp signals.

In normal mode, a mobile station searches for a control channel byfollowing a special control channel search sequence. A channel having ahigh enough field strength, the correct identifier information and whichresponses to the mobile station signalling is accepted as a controlchannel. In listening mode, the mobile station discontinues the searchsequence of a normal control channel and accepts the control channeleven if it does not receive a response to its signalling. The mobilestation, then, remains on the interfered channel, but listens to thedownlink channel transmitted by the base station, on which downlinkchannel there is no disturbance.

In listening mode, a mobile station does not have to transmitacknowledgement messages as a response to the system signalling, nortraffic channel information or speech. The mobile station can skip allthose procedures which are not essential as far as the reception ofdownlink traffic is concerned. Such procedures include, for example, allthe tasks required for the transmitting of a radio signal.

While in listening mode, a mobile station normally follows the systemcontrol, and, for example, shifts to conversation mode as it receives acall initialization message, and thus the mobile station goes to thetraffic channel on which a mobile services switching center or a basestation directs it.

The mobile communication system informs the mobile stations of jammingand sets the phones to listening mode. In such a case, the mobilestations do not have to register in order to hear downlink traffic. Thesystem dispatcher can give commands to the mobile stations within thearea even if it does not receive a response from them. Locating thejammer is also made easier, because it is the only device that istransmitting.

The mobile communication system returns the mobile station or theradiotelephones from listening mode to normal mode by transmitting afailure report message (FIG. 4) in which the value of the mode field 44is "normal mode". A mobile station can also be programmed to return tonormal mode after a predetermined time from the last failure reportcommand.

The mobile station, furthermore, returns to normal mode if its power isturned off; the phone always starts in normal mode.

It is, in addition, possible to return the mobile station from listeningmode to normal mode so that the mobile station is returned fromlistening mode to normal mode by a manual command given by the mobilestation user.

In cases wherein the signalling of the mobile station with the system isprevented although the mobile station is still able to receive signalsfrom the base station, the mobile station aims at finding out the reasonfor the break and informing the user of it. On the basis of failurereport information, or lack of it, the mobile station can infer whetherthe reason for the uplink disturbance is in the system, in the mobilestation itself, or in the differences between coverage areas. Theinference sequence is illustrated in FIG. 5.

In step 51 of the flow chart of FIG. 5, the mobile station determineswhether it can hear transmission from the system. If the mobile stationdoes not 52 hear it, the mobile station makes the estimate 53 that thedownlink traffic of the system in question is not operational as themobile station cannot hear the system. The reason for this can be thatthe base station transmitter does not work, or that the mobile stationreceiver does not work, or that the mobile station is outside of thecoverage area of the base station of the system. If, however, the mobilestation does hear 54 transmission from the system, the mobile stationestimates 55 whether the system responds to the signalling the mobilestation transmits. If the system responds 56, the mobile station knowsthat it can communicate with the system in the normal manner 57.

If, however, the system does not respond to the mobile stationsignalling 58 in an acceptable way, the mobile station begins to observe59 whether it receives a failure condition message. If the mobilestation does not 60 receive a failure condition message, it will know 67that the system is not faulty and that the system is not jammed. In sucha case, the mobile station knows that the uplink break is caused byreasons beyond control of the system. If, however, the mobile stationdoes receive 61 a failure condition message, the mobile station checks62 whether the failure condition message contains a command to shift tolistening mode. If the failure condition report does not contain 63 thefailure condition message, the operation of the mobile station proceedsas per step 67.

If, however, the failure condition message contains a command to shiftto listening mode 64, the mobile station shifts to listening mode 65 andknows that no transmission to the base station in acknowledgement of themessages it has received is necessary. In the same context, the mobilestation user is informed 66 similarly to above of the switch tolistening mode and the reason for it.

The drawings and their description are only meant to illustrate the ideaof the invention. The method of the invention for reacting todisturbance in a mobile communication system may vary in details withinthe scope of the claims. Although the invention is above describedespecially in connection with trunked mobile radio systems, theinvention can be utilized in other kinds of mobile communication systemsas well.

We claim:
 1. A method for reacting to disturbance on an uplink controlchannel in a digital trunked mobile communication system which includesa mobile communication network having a plurality of base stationsmaintaining a plurality of control channel pairs constituted by aplurality of uplink control channels respectively paired with acorresponding plurality of downlink control channels,a mobile servicesswitching center, at least one mobile station arranged for signallingwith a selected one of said base stations on a said control channel pairincluding a respective uplink control channel and a respective downlinkcontrol channel, said method comprising the steps of:(a) detecting bysaid mobile communication network of an instance of disturbance causedby intentional jamming on said respective uplink control channel; (b)informing said mobile station by said mobile communication network ofsaid disturbance, by use of said respective downlink control channel;(c) commanding of said mobile station by said mobile communicationnetwork to a listening mode, in which said mobile station is forced toremain exceptionally on said respective downlink control channel and isprevented from searching for a new, non-jammed control channel, eventhough said mobile station fails to receive any response to uplinksignalling transmitted by the mobile station to said selected one ofsaid base stations on said respective uplink control channels, andshifting of said mobile station to said listening mode; and (d)transmitting information to said mobile station by said selected one ofsaid base stations, and receiving said information by said mobilestation, on said respective downlink control channel, while said mobilestation is in said listening mode.
 2. The method of claim 1, wherein:inpracticing step (a), said disturbance is detected if a bit error ratioof signalling received at said selected one of said base stations onsaid respective uplink control frequency, at a predetermined fieldstrength, exceeds a corresponding bit error ratio at a correspondingfield strength previously experienced for signalling received at saidselected one of said base stations on said respective uplink controlfrequency in a situation known to have been substantially undisturbed.3. The method of claim 1, wherein:in practicing step (a), a digitaldemodulator of a receiver of said selected one of said base stationsdetects said disturbance on the basis of reception of signalling by themobile station on said respective uplink control channel behavingdifferently from normal.
 4. The method of claim 1, wherein:step (b)includes said mobile communication network transmitting a failure reportmessage to said mobile station.
 5. The method of claim 4, wherein:saidfailure report message includes information as to the intended meaningof said failure report message, information about said mobile station,information as to the nature of the disturbance which has been detectedin step (a), and a command for said mobile station to shift to saidlistening mode.
 6. The method of claim 5, further comprising:said mobilestation, based on reception thereby of said failure report message,providing information to a user of said mobile station as to saiddisturbance.
 7. The method of claim 4, further comprising:said mobilestation, based on reception thereby of said failure report message,providing information to a user of said mobile station as to saiddisturbance.
 8. The method of claim 7, wherein:said mobile stationincludes a display, and providing said information to said user includesproviding a text message on said display.
 9. The method of claim 7,wherein:said mobile station includes a sound emitter, and providing saidinformation to said user includes providing a sound signal.
 10. Themethod of claim 8, wherein:said mobile station includes an indicator 1amp and providing said information to said user includes providing avisually perceivable indication by said indicator lamp.
 11. The methodof claim 1, wherein:in practicing step (c), said shifting isaccomplished manually by a user of said mobile station, in response tosaid commanding.
 12. The method of claim 11, further comprising:beforepracticing of said shifting of step (c), checking whether said mobilestation has a right to shift to said listening mode.
 13. The method ofclaim 1, wherein:in practicing step (c), said shifting is accomplishedautomatically by said mobile station in response to said commanding. 14.The method of claim 1, further comprising: subsequent to practicing step(d), commanding of said mobile station by said mobile communicationnetwork to shift back from said listening mode to a normal mode ofoperating, and shifting of said mobile station back to said normal modeof operating.
 15. The method of claim 14, wherein:said shifting back isaccomplished manually by a user of said mobile station, in response tosaid commanding to shift back.